The present invention relates to image reject mixer circuits and in particular to image reject mixer circuits having a single ended input and two differential outputs. Radio frequency image reject mixers are very popular blocks of modern radio systems and are often used in preference to superheterodyne receivers, especially where frequency agility is required. The parameters of an image reject mixer determine the main characteristics of the system into which they are incorporated.
Prior art image reject mixers are generally based on the Gilbert cell or the micromixer circuit configuration. In the case of the Gilbert cell, an image reject mixer is compiled simply by connecting two Gilbert cell circuits in parallel. The input signal is split into two branches where they are mixed with an unshifted local oscillator signal and a 90.degree. shifted local oscillator signal respectively.
A section of an image reject mixer circuit based on the micromixer configuration is shown in FIG. 1.
In use, an input signal is applied to terminal 120, a local oscillator signal is applied to terminals 130, 131 and a 90.degree. phase shifted local oscillator signal is applied to terminals 132, 133. A reference potential is applied to terminal 125 to bias transistors 103 and 104. By virtue of resistors 150, 151, transistor 105 and biased transistor 103, a signal input at terminal 120 will give rise to complementary output current signals at the collector electrodes of transistors 103 and 101. Mixer core 160, formed by transistors 106-109, mixes the current signals from the collector electrodes of transistors 101 and 103 with the local oscillator signal applied to terminals 130, 131 and outputs a current signal at terminals 140, 141. Transistors 102 and 104 produce at their collector electrodes substantially the same current signal as is produced by corresponding transistor 101, 103 because these corresponding transistors are driven by the same input signal. Mixer core 170 mixes the current signal from the collector electrodes of transistors 102, 104 with a 90.degree. phase shifted local oscillator signal applied to terminals 132, 133 and outputs a current signal at terminals 142, 143. Because an unphased oscillator signal is applied to terminals 130, 131 and a 90.degree. phase shifted signal is applied to terminals 132, 133, output terminals 140, 141 will show an In-phase differential output and output terminals 142, 143 will show a Quadrature differential output.
As will be appreciated, the mixer circuit shown in FIG. 1 is incomplete. The full mixer circuit implementation would also have means for phase-shifting the output of mixer core 170 by 90.degree. and summing the resultant signal with the output from mixer core 160. This would result in either the image band signal or the signal band signal as the complete mixer circuit output, depending on the sign of the 90.degree. phase-shift imposed on the signal output from mixer core 170.
Whilst the image reject mixer circuit section of FIG. 1 has a wide dynamic range and very linear operation, the presence of so many resistors gives the mixer circuit very poor noise properties.
Image reject mixers constructed from Gilbert cell circuits have poor noise properties due to resistors in the main current paths, current sources experiencing high frequency, large voltage swings and poor transistor arrangements. It is difficult also to design an image reject mixer using Gilbert cell circuits so that it has a particular input impedance. This can be a drawback when impedance matching with a pre-amp stage is necessary. There exists a need for an image reject mixer circuit with improved noise